Method of making a magnetic material part with spatial distribution of the permeability

ABSTRACT

A part made of ferrimagnetic material with a spatial variation of the permeability value is obtained through heat treatment of a body of ferrimagnetic material at the temperture such that the free energy of the material is low and that a transformation into a material with higher free energy is obtained, stopping said heating before complete transformation and further machining of the part. The heat treatment is carried out under reduced pressure when a liquid phase develops at the heating temperature. When solid phase is maintained the body is heated in presence of a metal salt or oxide.

United States Patent 1191' Deschamps et al.

[ Dec. 24, 1974 'Feoxme METHOD OF MAKING A MAGNETIC inmol% [54]3,057,802 10/1962 Pierrot et al 252 6262 MATERIAL PART WITH SPATIAL3,132,105 5/1964 Harrison et a1. 252/6257 3,457,174 7/1969 Deschamps eta1 252/6256 X DISTRIBUTION OF THE PERMEABILITY 3,763,045 10/1973Takamizawa et al. 252/6257 '[75] Inventors: Andi- Deschamps; GeorgesFaye,

both of Pans France Primary ExaminerCarl E. Hall [73] Assignee: SocieteLignes Telegraphiques Et Attorney, Agent, or Firml(em0n, Palmer &

Telephoniques, Paris, France Estabrook 22 Filed: Mar. 2, 1973 i [21]Appl. No.: 337,308 [57] ABSTRACT A part made of ferrimagnetic materialwith a spatial Forelgn Apphcatlon Prlomy Data variation of thepermeability value is obtained through Mar. 7, 1972 France 72.07816 heattreatment of a body of ferrimagnetic material at Mar. 28, 1972 France72.10779 the temperture such that the free energy of the material is lowand that a transformation into a material [52] US. Cl 29/608, 252/6257,252/6262, with higher free energy is obtained, stopping said heat-336/233 ing before complete transformation and further ma- [51] Int. Cl.H01f 3/08 chining of the part. The heat treatment is carried out [58]Field of Search 29/608, 607, 602; under reduced pressure when a liquidphase develops 252/6256, 62.57, 6262; 336/233 at the heatingtemperature. When solid phase is maintained the body is heated inpresence of a metal salt or [56] References Cited oxide.

UNITED STATES PATENTS 10 C 14 D F. 2,982,948 5/1961 Brownlow CI al.336/233 ux raw'ng gums v 6 C Mognenre LlqUldUS Perovskire I B+u moo qG'ornet llq M H G 1 ogne e orne N ,O m g Ln L. l300 93 8 5Hemo1|1e+Gornet 50 g Y Oxide .1 7 METHOD OF MAKING A MAGNETIC MATERIALPART WITH SPATIAL DISTRIBUTION OF THE PERMEABILITY SUMMARY oF THEINVENTION The present invention concerns a process for the preparationof homogeneous articles of the ceramic type, of which the magneticproperties vary from point to point. Magnetic circuits whosepermeability varies spatially are usually built up of differentmaterials chosen for the appropriate value of their permeability.

However, the assembly of various materials may present disadvantagesand/r difficulties, for instance when the dimensions of the articles are,small'or when it is likely to be subjected to such temperaturedifferences that harmful stresses will develop in the assembly ofmaterials due todifferences in expansion coefficients.

. 2. a chemical composition differing from that of the solid and thethird case is achieved through solid phase diffusion of a salt or oxideof the substituting ion into the isting in the structure does not modifythe chemical machining of said piece to obtain the required spatialdistribution.

BACKGROUND OF THE INVENTION It is well known that the magneticproperties of mate-' rials relies on the algebraic sum of'the spins ofthe ions distributed between the various crystallographic sites.

When this sum is zero, the material is non-magnetic (its permeability isnear that of the vacuum). Change of ion in the sites may change thevalue of the sum and therefore of the permeability. In practice, softmagnetic materials have been obtained essentially from structures of thespinel type, that is to say, those belonging to the space group 0;, F 3dm, and from structures of the garnet type, belonging to the group 0;, la 3 d.

From the physico-chemical viewpoint, the change may be summed up eitheras a suppression of ions from some sites, as the introduction of foreignions into the lattice or as a substitution of ions at some of thelattice sites. The second case will not be taken into consideration dueto the fact that too little vacancies are available in current softmagnetic materials for easy fill up of the lattice introducing animportant change in the magnetic properties. The invention therefore isbased on the first and third type of modifications. According to theinvention, the first case consisting of locally depriving the lattice ofions through heating the material will be used when the phase diagram ofthe material shows the formation of a liquid phase corresponding tocomposition and may result in a deformation of the crystal lattice, andthe ion driven off is accumulated in the form of an oxide or of anothersalt at the grain boundaries; in the second, the substitution reactionis accompanied by a modification of chemical composition since theprevious ions are evaporated as oxide or salt or the substitution isonly a change in the valency of a polyvalent metal (such as iron forinstance) with evaporation of molecules corresponding to the anions ofthe lattice.

The structural changes within the lattice of the material are based onthermodynamic equilibrium as is briefly summarized below. In fact, thesetwo aspects are different embodiments of the same physicochernicalmechanism.

For any stable chemical composition at the temperature T underconsideration, the free energy of the material is given by the absolutevalue of the expression:

where U is the internal energy S is the entropy, and T is the absolutetemperature. If the vibration energy and the magnetic interaction energyare disregarded, U appears as the sum of:

-. U, electrostatic energy U, Borns elastic repulsion energy U covalentbond energy I As is known, entropy is linked to the presence of disorder(S is low for ahighly ordered structure); since a temperature riseresults in a reduction of the order of the crystal lattice (specially ifa liquid phase develops), the entropy increases with the temperature.The product TS therefore increases with temperature. In order toevaluate U, it is necessary to make an hypothesis on the nature of thematerial. Let us consider, for example, a spinel. The general formula ofspinels indicating the distribution of the metallic ions between thetetrahedral sites A (8 metallic ions per mesh) and the octahedral sitesB (16 metallic ions per mesh), is written:

. Octahedra]. sites B where M represents a bivalent ion. The position ofthe 32 oxygen ions is fixed by the parameter 8 as defined in theinternational crystallographic tables. The unit cell of such a spinelstructure is shown and explained at page in the book entitled Microwaveferrities and ferrimagnetics. by Lax and Button published by Mac GrawHill in 1962.

The electrostatic energy U, of a spinel is expressed as a function ofthe distribution of the ions between the sites A and B (parameter A) andof the position of the oxygens (parameter u 3/8 8). )t and 6 aredetermined for each structure by X-ray analysis. The usual values of 8are comprised within the range (0 8 0.01 The repulsion energy U, can becalculated from the Born-Mayer formula. The covalence energy U is zeroin the absence of a covalent bond. When covalent bonds exist, U may behigh and, as a result of the decrease of the ionic charge, it may becomethe preponderant factor of U.

In the case of garnets, the internal energy is also linked to thestructure. However a different phenomenon occurs due to the phasediagram of yttrium-and iron oxides the system (yttrium iron garnet)shown on FIG. 1 as published in the Journal of the American CeramicSociety, volume 5, no. 44, page 213.

If a garnet material (supposed stoichiometric in composition) is heated,the material will follow line AB of the diagram at the crossing of theliquidus horizontal line (T 1510 C). E is quite small. If the freeenergy of perovskite is considered at the same temperature (line CE), noliquid phase is present and the free energy is much higher. Thisexplains that a structural change occurs spontaneously part of thegarnet being changed into perovskite. From the chemical viewpoint thischange can be written as:

3 Y O 5 F6203: 3 Y O 3 F6203 2 F8203 (garnet solid) (perovskite solid)(liquid) Since the heating energy is high enough, reduction of theliquid iron oxide occurs 'simultaneuosly:

' 3F62O3 2Fe0 2F62O3 I02 (liquid) (liquid) (liquid) As shown one out ofthree trivalent iron ions becomes divalent. These divalent ions givebirth to a spinel structure Fe O FeO which is liquid and therefore has asmall free energy value. The two last reactions do not cooperate in anhigh proportion to the free energy of the material at point B. The freeenergy at B of the liquid garnet phase is much smaller than the freeenergy at the same temperature of the solid perovskite phase.

According to the invention the change in the magnetic properties of amagnetic material F is obtained by heating a piece of said magneticmaterial at a temperature such that the free energy of the F material ismuch smaller than the free energy of another ferrimagnetic material Finto which F is transformed at said temperature, this change is stoppedbefore it has been carried out completely and the piece of material ismachined so that at least one part of the outer surface of said machinedpiece consists of the initial F material.

According to a first variant of the invention, a body of a material ofcomplex structure having the magnetic properties required to constitutea fraction of the solid magnetic article is subjected to a thermaltreatment in vacuo at a temperature several degrees higher than thesolidus-liquidus equilibrium temperature that is at a temperature wherethe free energy is near zero for a period such that the magneticproperties of the external fraction of the body only are modified byevaporation of the liquid phase, the initial magnetic properties of thecentral fraction of the body being retained. By subsequent machining ofthe body thus treated, there can be obtained a mechanically solidmagnetic article having the desired space distribution of permeabilityvalues. The application of a thermal treatment in vacuo facilitates theevaporation of the liquid phase formed.

The use of a temperature which is several degrees higher than thetemperature at which the liquid phase is formed renders possible agradual transformation of the structure of the material which retainsthe mechanical cohesion of the body. This vvariant corresponds roughlyto an out-diffusion according to solid state physics terminology sincethe liquid phase which is evaporated carries of some of the lattice ionsand thereby changes the sum of the spins in the lattice. Sometimesfiirther chemical changes simultaneously occur as explained withreference to the yttrium, iron oxides diagram.

According to a second variant of the invention in solid phase, adiffusion is effected in a crystal structure of an initial magneticmaterial, that is to say, the replacement of an ion M in one site by adifferent ion M corresponding to a second structure of magnetic materialby heating the material in presence of a M salt or oxide at atemperature such that E E where E, is the internal energy of theinitial'maten'al (M Fe O and E is the energy of the second material (MFe O and such that E is very low. In order terms, U TS U, T5,.Experience has shown that this temperature is higher than 850 C for thespinels usually used as soft magnetic materials and about l200 C in thecase of garnets.

As will be apparent from the following description of the examples, thethermal treatment may have the ef fect of increasing or reducing thepermeability of the material, depending upon the initial composition ofthe latter. Similar treatments may have either effect. Thetemperature'of the thermal treatment is fixed by the above conditionsfor the free energy. In case of the liquid phase intermediate it can beselected on the phase diagram of the constituents of the body. Theapplication of a reduced pressure in the course of the thermal treatmentgenerally results in a reduction of the temperature at which the liquidphase is formed, as compared with its value at atmospheric pressure.This point is particularly favourable to maintenance of the mechanicalcohesion of the body.

DETAILED DESCRIPTION OF THE DISCLOSURE The invention will be readilyunderstood from the following description, which is given by wayofnonlimiting illustration, and with reference to the accompanyingfigures, in which:

FIG. 1 is a known phase diagram of the mixture of iron and yttriumoxides,

FIG. 2 shows experimental curves representing the value of thesaturation magnetisation as a function of the space co-ordinates of acylindrical block of an ironyttrium-aluminium-gadolinium garnet treatedin accordance with the invention,

FIG. 3 is a diagram of the block of material corresponding to the abovecurve,

FIGS. 4 to 7 inclusive are micrographic views of different sections ofthe material from which the curves of FIG. 2 were obtained,

FIG. 8 is a diagram representing the saturation magnetisation offerrites of Ni and Zn as a function of the composition, and

FIGS. 9 and 10 are micrographs of a specimen of Ni-Zn ferrite treated inaccordance with the invention,

FIG. 11 is a nickel ferrite specimen which has undergone a heattreatment,

ium", of a type marketed under No. 6,905 by the Applicants. Thismaterial, which is green in colour, has good magnetic properties (41rMsof the order of 900 to 1,000), and is particularly adapted to operate inthe frequency range of 9GHz (linewidth between 70 and 80 oerstedstangent of the loss angle of the order of I 10"). It is commonly used inthe production of circulators, isolators, etc., based upon thepropagation of elec tromagnetic microwaves in a high-permeabilitymaterial. It is well known to every specialist that the use of such agarnet as a substrate for integrated microwave circuits is hindered bydisadvantages which could be at least partially eliminated by areduction of the permeability of the ferrite at the places where nomagnetic field is applied. The geometrical distribution of thepermeability values depends, of course, upon thedesign of circuit to beproduced. a

' As already mentioned, FIG. 1 shows the phase diagram, at atmosphericpressure, of the iron oxideyttrium oxide mixture, as published in theJournal of American Ceramic Society 1961 Vol. 5, No. 44, page 2l 3. Thequaternary diagram of the iron-yttriumgadolinium-aluminium oxidemixtures cannot be represented in a simple form on the same diagram.Since aluminium oxide is more refractory than iron and yttrium oxidesthe appearance of an aluminium-rich liquid phase takes place only attemperature higher than those at which the iron (or yttrium) oxideliquid phase appears. At the temperatures indicated in the diagram ofFIG. 1", the presence of the other two constituents has scarcely anyeffect on the phase diagram. In accordance with the present invention,there is employed as the initial material a solid body, for example ofcylindrical form, consisting of a garnet obtained by any known method,such as for instance by that described in the British application filedby the Applicants on theFeb. 2nd, 1973 for: Improved production processof fine grained ferrites, which body is coated in alumina powder anddisposed in a vacuum oven whose temperature is brought to l,300 C afterout-gassing for about 1 hour at a pressure of 10' cm.Hg. The temperaturerise takes place in about minutes. The garnet follows a course similarto AB on the phase diagram. At the point B, there are present a solidphase consisting of garnet transformed into perovskite (or orthoferrite)and a liquid phase consisting essentially of iron oxide. At l,290 C,there is noted the appearance of a liquid phase which is at leastpartially absorbed by alumina. The

temperature is maintained for 3 hours at 10 C above the point at-whichthe liquid phase-appears. The oven is then cooled by natural cooling(about 2 hours). When the body is removed from the oven, it is foundthat the colour of the external layer has become brownish-red, whichclearly corresponds to the colour of perovskite.

The curves of FIG. 2 show the variation of the saturation magnetisationof the body subjected to two successive series of machining operations,the first being a plane grinding having the effect of eliminatingfragments parallel to the base of the cylinder and reducing the heightof the body, while the second is a cylindrical grinding having theobject of eliminating a cylindrical film and reducing the diameter ofthe body.

' FIG. 3 illustrates these successive operations. The overall saturationmagnetisation of the article after thermal treatment is gauss/cm. In thecourse of a first series of steps, discs such as l, 2, etc. in adirection parallel to one of the terminal faces of the cylinder aresuccessively eliminated. These successivediscs have a thickness of 1 mm.The initial height of the article is 10 mm and its diameter is 2.75 mm.As will be apparent from FIG.'2, the elimination of a fragment of 1 mm,such as the fragment 1' at one of the ends of the cylinder results in aconsiderable increase in the saturation magnetisation, which changes to735 gauss/cm (origin of the curve h 9 mm). The successive eliminationsof fragments of 1 mm, such as l, 2, 3, 4, results in the case of thefollowing two fragments in an increase of the saturation magnetisation,which changesto 745 gauss/cm for 6 mm, and 775 gauss/cm for 5 mm,respectively. It consequently appears clear that the first fragmentseliminated had no magnetic property, the high increase in the saturationmagnetisation being a result of a reduction in volume due to eliminationof a non-magnetic fraction of this body. The elimination of thesuccessive fractions continues to increase the saturation magnetisation,but in much smaller proportions, which would tend to show that thevolume of the material eliminated has a permeability lower than maximumpermeability. The elimination of the last section, bringing the heightof the body to 4 mm, even results in a levelling out of the saturationmagnetisation, taking into account the precision of the measurements.This result would tend to indicate that the material removed in thecourse of this last operation has a mean permeability similar to that ofthe remaining body. It will be observed that this last section reachesthe centre of the treated cylindrical body. A second series of steps wascarried out on the same body by cylindrical grinding of the remainingbody, i.e. by reductionof the diameter. The results obtained are shownin the upper part of the curve of FIG. 2. As will be apparent, theelimination of the external zone of the cylinder results in anappreciable increase in the saturation magnetisation, which confirmsthat only material of low permeability has been removed. With'an initialdiameter of 27.5 mm, a saturation magnetisation of 760 gauss/cm wasmeasured. The saturation magnetisation measuredis 850 gauss/cm for adiameter of 25.5 mm. The measurements made at 23.5 mm and 2L5 mm givevalues of 777 I and 1030 gauss/cm. The last measurement made on thearticle reduced to a diameter of 10 mm gives a saturation magnetisationof 1,060 corresponding to the value of the saturation magnetisation ofthe garnet before heat treatment. The curve of FIG. 2 therefore showsthe superficial modification of the permeability by heat treatment ofthe garnet and the possibility of maintaining in a part of a solid bodythe initial values, while notably modifying the penneability values in aperipheral zone. The experimental results just described thus make itpossible to obtain, by plane grinding of cylindrical articles, discs ofhomogeneous material whose centre, of green colour, has the magneticcharacteristics of the garnet, and in which a red-coloured peripheralzone has low permeability and good dielectric properties. The mechanicalcharacteristics of the wafer thus obtained are entirely similar to thoseof a garnet wafer.

FIG. 4 is a micrographic view of the section of the body of FIG. 2before the heat treatment, with a magnification of 50. The lower partcorresponds to the peripheral zone and has relatively coarse grains. Thecentral part of the micrograph corresponds to a bright zone and theupper part corresponds to the centre of the body and to a green zone.Larger-scale micrographs of the three zones appearing in FIG. 4 form thesubject of FIGS. 5, 6 and 7 which correspond respectively to theperipheral zone, to the bright intermediate zone and to the centralzone. The magnification of these micrographs is 1,000. As will beapparent, the dimensions of the grains constituting the peripheral zoneare much greater than those of the grains constituting the central zone.The black portions of the photographs correspond to grains torn out inthe course of the polishing treatments and to the chemically attackedgrain junctions. The average dimensions of the grains change from 20microns in the peripheral zone to 6 microns in the central zone. Therewill be observed in the central zone continuous dark paths which may beinterpreted as diffusion channels and which would tend to show that theheat treatment carried out has started to modify the 'core of the body.There is observed in the course of the heat treatment an evaporation ofa liquid iron oxide phase progressively absorbed by the alumina powderin which the body is buried.

Zn, F620 for various compositions (variation of 8 between 0.65

and 0.00). This diagram is taken from the book Les ferrites by Smit andVijn, published in 1959 by Philips Technical Library, p. 158.

As will be seen, at C (points A, B, C, D), the saturation magnetisationvaries with the zinc concentration. When the starting material contains65 percent of Zn (point A), the reduction in the ZnO content results ina slight increase in the saturation magnetisation (points B and C),followed by a reduction (point D corresponding to pure Ni ferrite). Thefollowing experiments have shown that in fact the reduction of ZnOcontent which can be obtained by thermal treatment in vacuo of thematerial results, in accordance the heating temperature, in arearrangement of the structure of the ferrite, accompanied by anelimination of oxygen or ferric oxide. The two following experimentswere carried out on a ferrite corresponding to the point B of thediagram at 6 0.2.

In a first experiment, the ferrite is heated in vacuo for 2 hoursbetween l,l00 and l,250 C in an alumina crucible. There is observed inthe course of the thermal treatment aliberation of zinc oxide vapour anda liberation of oxygen. The reaction may be written as follows:

The starting ferrite has a saturation magnetisation of 4760 gauss/cmwhich value is reduced to 3800 gauss/cm after the thermal treatment. Themeasured loss angles show that the body thus obtained cannot be used ina microwave circuit owing to the presence of F8304. 7

When the same ferrite is heated to temperatures between l,270 and 1,3 10C, there is observed an evaporation of zinc oxide, followed by theappearance of a liquid iron oxide phase, which evaporates. The reactionthen occurring is the following:

0.8Ni0, 0.2Zi10, Fe20a and there are in fact obtained, after thermaltreatment, approximately the characteristics of the pure nickel ferriteboth for the value of the saturation magnetisation (3,600 gauss/cm andfor that of the losses.

Experience has shown that heating above the temperature defined in thesecond experiment also degrades the characteristics of the productobtained, owing to the formation of Fe O In a first series ofsubstitution experiments, the initial material is a nickel ferrite ofspinel structure, NiFe O in which 2 l, the nickel ions occupying onlythe octahedral sites B. Cupric ions are diffused into this structureunder the following conditions: after sintering and optional grinding,the spinel is disposed in a non-porous sintered and crystallised aluminaboat provided with a lid. A few grammes of copper oxide CuO are addedand the temperature is raised to 900 C and maintained for 48 hours. Thesaturation magnetisation of the material after treatment issubstantially nil. Using the above notations, it is known that copperoxide CuFe O has a structure such that 2)\ 0.88 and 8 0.005. The copperions occupy the octahedral sites in a majority, and they have strongsquare covalent bonds in the spinel structure, which even produce aquadratic deformation of the mesh. The covalence energy of copper istherefore very high. Nickel ferrite does not exhibit any covalent bonds.If the nickel ferrite is denoted by F, and the copper ferrite by F thenat 900 C E, is in the neighbourhood of zero, while the free energy E isdistinctly higher.

In the course of the substitution reaction, the nickel ions becomeconcentrated in the form of oxide in the grain boundaries. In anotherseries of substitution experiments, nickel ferrite was treated under thesame conditions at l,250 C for 48 hours in the presence of zinc oxide.There was again obtained a substitution of the nickel ions by Zinc ions.Under the above conditions, an increase of the saturation magnetisationof the nickel ferrite from 3,300 to 4,300 gauss/cc was obtained. It isknown that, in zinc ferrite, the zinc ions occupy the tetrahedral sitesA (2% 0), while the nickel ions of the initial structure occupy thesites B. The diffusion is therefore accompanied by a transfer of themetallic ions in addition to their substitution. The zinc ions in thefinal structure exhibit strong tetrahedral covalent bonds, whichexplains why the free energy of the zinc ferrite is higher at l,250 Cthan that of nickel ferrite. Account must also be taken, in evaluatingthe free energy, of the energy variation resulting from the transfer ofthe ions between the sites B and A, which variation may be evaluated, asa first approximation, by the electrostatic energy difference between anormal and 9 an inverted spinel (cf the above mentioned book by Lax andButton page 115).

The same nickel ferrite was subjected to the same experimentalconditions in the presence of boron oxide and heated at l,300 C for 48hours. The saturation the electrostatic attraction forces are higherthan in the case of nickel. The component U, of the internal energy isincreased at a given temperature, which clearly corresponds to a valueof E E,. The diffusion temperature is so chosen that the nickel ferritehas under these conditions a low internal energy. The micrograph showsthat the grain junctions have substantially disappeared. Themagnification of the latter micrograph is 150, and that of the precedingones 1,000.

The yttrium-gadolinium-aluminium garnet marketed under the number 6905by the Applicants, and having a saturation magnetisation of 850gauss/cc, was treated for 48 hours at l,300 C in a closed boat in thepresence of boric oxide. After treatment, the saturation magnetisationhad become 400 gauss/cc. This diffusion is accompanied, as in theprevious case, by a change of chemical composition due to formation ofyttrium borate YBO As in the previous case, the electrostatic energy ofthe borate is substantially higher than that of the garnet. The freeenergy of the second ferrite is therefore considerable.

The foregoing examples describe diffusion treatments with oxide. Saltssuch as chlorides or nitrates may also be used. However, it is oftensimpler to choose oxides so as to avoid the development of highlyreactive gas which may accompany the decomposition of the salts.

What we claim:

1. A process for the manufacture of a solid magnetic article with aspatially uneven distribution of permeability which comprises:

providing a sintered solid piece of polycrystalline magneticallyuncompensated initial F material having a first permeability value,

heating said sintered solid piece at a temperature such that the freeenergy of said F, material is smaller than the free energy of anotherferrimagnetic F material having a second permeability value differentfrom said first permeability value,

transforming at said temperature a portion of said F material into saidF material,

discontinuing said heating before the material of said piece iscompletely transformed into F material, and

2. A process for the manufacture of a solid magnetic article with aspatially uneven distribution of the permeability according to claim 1in which a liquid phase of different chemical composition develops atthe heating temperature, the chemical composition of which differs fromthat of the initial l material.

3. A process according to claim 2 in which said heating is made underreduced pressure.

4. A process for the manufacture of a solid magnetic article with aspatially uneven distribution of permeability according to claim 3 inwhich said initial F material is an yttrium iron Gd, Al substitutedgarnet and said heating temperature is between l,250 and 1350 C.

5. A process for the manufacture of a solid magnetic article with aspatially uneven distribution of permeability according to claim 3 inwhich said initial F material is a Ni Zn spine] ferrite and said heatingtemperature is between 1,270 and l,3l0 C.

6. A process for the manufacture of a solid magnetic article with aspatially uneven distribution of the permeability according to claim 1in which said heating is carried on at a temperature lower than theliquefaction of the F material in presence of a salt of a foreign metalwhich constitutes the F material through substitution.

7. A process for the manufacture of a solid magnetic article with aspatially uneven permeability distribution according to claim 6 in whichsaid initial material is nickel spinel, the metal ion of said salt offoreign metal is divalent zinc and said heating temperature is above 850C.

8. A process for the manufacture of a solid magnetic article with aspatially uneven permeability distribution according to claim 6 in whichsaid initial material is nickel spinel, the metal ion of said salt offoreign metal is divalent copper and said heating temperature is above850" C.

9. A process for the manufacture of a solid magnetic article with aspatially uneven permeability distribution according to claim 6 in whichsaid initial material is nickel spinel, the metal ion of said salt offoreign metal is divalent boron and said heating temperature is above850 C.

10. A process for the manufacture of a solid magnetic article with aspatially uneven permeability distribution according to claim 6 in whichsaid initial material is an yttrium-iron garnet the metal ion of saidsalt of foreign metal is divalent boron and said heating temperature isabove l,200 C. I

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,855,691Dated December 24, 1974- Invent0r(s) Andre Deschamps and George Faye Itis'certified that error appears 'in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, line 20, read --Inother terms-- instead of I I "'In orderterms" Column 10, line 10, read --of the initial F material-- instead of'of the initial I material" Signed and sealed this 1st day of April1975.

fittest:

C. MARSHALL DANN Commissioner of Patents and Trademarks RUTH 6. EMS Nf." Officer FORM PO-l050 (10-69) U5cOMM- c 50375-1569 1 us. sovlsunimmanna OFFICE: nu o-su-su.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,855,691 Dated December 4 Inventor(s) Andre Deschamps and George FayeIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 4, line 20, read -In other termsinstead of "In order terms".

Column 10, line 10, read -of the initial F material-- instead of "of theinitial I material".

Signed and sealed this 1st day of April 1975.

(SEAL) jttest:

firm" rm C. I'EARSHALL DANN 1:. Lil Commissioner of Patents .-Ittest' ngOluflCeI and Trademarks 'ORM PO-10 (1 uscoMM-Dc 60378-P69 U.S.GOVERNMENT PRINTING OFFICE ID, 0-36-33,

1. A PROCESS FOR THE MANUFACTURE OF A SOLID MAGNETIC ARTICLE WITH ASPATIALLY UNEVEN DISTRIBUTION OF PERMEABILITY WHICH COMPRISES: PROVIDINGA SINTERED SOLID PIECE OF POLYCRYSTALLINE MAGNETICALLY UNCOMPENSATEDINITIAL F1 MATERIAL HAVING A FIRST PERMEABILITY VALUE HEATING SAIDSINTERED SOLID PIECE AT A TEMPERATURE SUCH THAT THE FREE ENERGY OF SAIDF1 MATERIAL IS SMALLER THAN THE FREE ENERGY OF ANOTHER FERRIMAGNETIC F2MATERIAL HAVING A SECOND PERMEABILITY VALUE DIFFERENT FROM SAID FIRSTPERMEABILITY VALUE, TRANSFORMING AT SAID TEMPERAURE A PORTION OF SAID F1MATERIAL INTO SAID F2 MATERIAL,
 2. A process for the manufacture of asolid magnetic article with a spatially uneven distribution of thepermeability according to claim 1 in which a liquid phase of differentchemical composition develops at the heating temperature, the chemicalcomposition of which differs from that of the initial I1 material.
 3. Aprocess according to claim 2 in which said heating is made under reducedpressure.
 4. A process for the manufacture of a solid magnetic articlewith a spatially uneven distribution of permeability according to claim3 in which said initial F1 material is an yttrium iron Gd, Alsubstituted garnet and said heating temperature is between 1, 250* and1350* C.
 5. A process for the manufacture of a solid magnetic articlewith a spatially uneven distribution of permeability according to claim3 in which said initial F1 material is a Ni - Zn spinel ferrite and saidheating temperature is between 1,270* and 1, 310* C.
 6. A process forthe manufacture of a solid magnetic article with a spatially unevendistribution of the permeability according to claim 1 in which saidheating is carried on at a temperature lower than the liquefaction ofthe F1 material in presence of a salt of a foreign metal whichconstitutes the F2 material through substitution.
 7. A process for themanufacture of a solid magnetic article with a spatially unevenpermeability distribution according to claim 6 in which said initialmaterial is nickel spinel, the metal ion of said salt of foreign metalis divalent zinc and said heating temperature is above 850* C.
 8. Aprocess for the manufacture of a solid magnetic article with a spatiallyuneven permeability distribution according to claim 6 in which saidinitial material is nickel spinel, the metal ion of said salt of foreignmetal is divalent copper and said heating temperature is above 850* C.9. A process for the manufacture of a solid magnetic article with aspatially uneven permeability distribution according to claim 6 in whichsaid initial material is nickel spinel, the metal ion of said salt offoreign metal is divalent boron and said heating temperature is above850* C.
 10. A process for the manufacture of a solid magnetic articlewith a spatially uneven permeability distribution according to claim 6in which said initial material is an yttrium-iron garnet the metal ionof said salt of foreign metal is divalent boron and said heatingtemperature is above 1,200* C.